A microstrip antenna is a planar antenna in which a feed line extending with a substantially constant width and radiating elements excited by a traveling wave propagating through the feed line are formed on a dielectric substrate, and fed with power using a waveguide or the like. The feed line is a microstrip line configured to include a microstrip conductor formed on the front surface of the dielectric substrate and a grounding plate formed on the back surface of the dielectric substrate. Such a planar antenna uses a distributor in order to distribute high frequency power according to the number of radiating elements. The distributor is a power distributing circuit adapted to distribute the high frequency power fed from a feeding point to a branching point through an input line to two or more output lines (see, for example, Patent Literatures 1 and 2).